: Biocatalysts play crucial roles in the construction of therapeutics, and have the additional potential to serve as novel classes of therapeutics by modifying disease-associated proteins. Despite their significant advantages, the use of enzyme catalysts in industry and academic research has been limited by their narrow substrate scope. Directed evolution methods have demonstrated the ability to engineer specific enzymes to adopt new functions not present in the wild-type enzymes, however, conventional laboratory evolution strategies are time- consuming and their application to a variety of new enzyme functions has proven challenging. Here I propose two approaches to evolve bond-forming catalysts, using phage-assisted continuous evolution (PACE) or yeast display-based bond-formation selection, two powerful directed evolution platforms recently developed by the Liu group. The evolution of bond-forming catalysts will permit the discovery of new enzymes with tailor-made substrate scopes. As an initial application of these methods and concepts, we are evolving variants of sortase, a transamidase enzyme, to catalyze bond-formation events with tailor-made specificities, including specificities for disease-associated proteins. Our preliminary results suggest the successful evolution of bond-forming enzymes with novel substrate specificities.